This invention concerns, in general and according to one of its aspects, a separation process making it possible to separate the constituents of natural gas into a first gas fraction which is rich in methane and essentially depleted of C2 and higher hydrocarbons, and a second gas fraction, which is rich in C2 and higher hydrocarbons and essentially methane-depleted.
A process of the kind to which the invention is directed is known in the prior art as shown by U.S. Pat. No. 5,881,569.
Ethane contained in natural gas can be extracted with known processes, as described in U.S. Pat. Nos. 4,140,504; 4,157,904; 4,171,964; and 4,278,547. Although the processes described in these patents are of interest, in practical terms they allow, at best, an ethane recovery rate of about 85%. They use liquid/gas separators, heat exchangers, pressure reducers (usually in the form of turbines), compressors, and distillation columns.
More recently, other processes have been made public, particularly by U.S. Pat. Nos. 4,649,063; 4,854,955; 5,555,748; and 5,568,737. Although these more recent processes can result in relatively satisfactory extraction yields of ethane and other hydrocarbons, in order to obtain fractions rich in methane or C2 and higher hydrocarbons, these processes require relatively substantial energy consumption.
In view of this, the present invention is designed to reduce energy consumption in the production of fractions rich in methane or C2 and higher hydrocarbons, while maintaining much higher extraction yields than the processes of the prior art.
The invention concerns, according to one aspect, a process for separation of a mixture that is cooled under pressure and that contains methane, C2, and higher hydrocarbons, into a final light fraction rich in methane and a final heavy fraction rich in C2 and higher hydrocarbons, comprising a first stage in which the cooled mixture is separated under pressure in a first flask, into a first top fraction which is relatively more volatile, and a first bottom fraction which is relatively less volatile, in which the first bottom fraction is introduced into the middle part of a distillation column in which there is collected, in a lower part of the column, as a second bottom fraction, the final heavy fraction rich in C2 and higher hydrocarbons, in which there is introduced, after it has been reduced in pressure in a turbine, the first top fraction in an upper part of the distillation column, in which there is collected, in the upper part of the column, a second top fraction rich in methane, in which the second top fraction is then subjected to compression and cooling to obtain the final light fraction, and in which a first sample fraction is taken from the final light fraction, this process including a second stage in which the first sample fraction is introduced, after cooling and liquefaction, into the upper part of the distillation column.
The process of the invention, includes a third stage in which the first bottom fraction is subjected to a number of sub-stages including reheating, passage into a second flask, and separation into a third top fraction which is relatively more volatile and a third bottom fraction which is relatively less volatile, in which the third bottom fraction is introduced into the middle part of the distillation column, and in which the third top fraction, after cooling and liquefaction, is introduced into the upper part of the distillation column.
Another process, as described in U.S. Pat. No. 5,566,554, uses two liquid/gas separators, of which a liquid fraction collected at the bottom of the first separator is heated then introduced into a second separator. This technique makes it possible, in particular, to improve the extraction of the methane contained in the bottom fraction from the first separator, and especially to use the pressure reduction of this bottom fraction to cool the natural gas stream to be treated which is entering the installation, in a heat exchanger.
On the other hand, this known process cannot be used to obtain extensive extraction of ethane, because the quantity of reflux generated by the technique is low, and the ethane content of this reflux is relatively high.
The present invention overcomes these problems by two means.
First, the invention provides for diversion of part of the methane-rich fraction at the top of the column and its reintroduction into the last stage of the column after compression and cooling. This makes it possible to obtain a reflux in sufficient quantity and of excellent quality, as the C3 content is very low, for example, less than 0.1 mol %.
Second, the invention provides for diversion to the column of part of the first top fraction from the first separator before the stage of pressure reduction in the turbine. This second diverted fraction is cooled and liquefied before it is introduced into the column. This method of proceeding limits the quantity of recycle and liquefied gas mentioned above and reduces the related compression costs.
The invention may also provide for removing a second sample fraction from the top fraction, and introducing this second sample fraction, after cooling and liquefaction, into the top of the distillation column.
According to one possible embodiment of the invention, the second sample fraction is cooled and partly condensed, then separated in a third flask into a fourth relatively more volatile top fraction, which is cooled and liquefied, then introduced into the upper part of the distillation column, and into a fourth relatively less volatile bottom fraction, which is heated, then separated in a fourth flask into a fifth relatively more volatile top fraction which is cooled and liquefied, then introduced into the upper part of the distillation column, and a fifth relatively less volatile bottom fraction which is heated and then sent into the second flask.
The invention may also provide that the lower part of the distillation column should comprise a number of stages connected in pairs to one or more lateral reboilers.
The invention may also provide that, to obtain the final light fraction, after the second top fraction leaves the distillation column, the latter undergoes reheating, a first compression in a first compressor connected to the pressure reduction turbine, a second compression in a second compressor, and cooling.
The invention may also provide that the upper part of the distillation column also comprises at least two successive stages, the first of which is the lowest, and that the fifth top fraction be introduced above the first stage.
The invention may further provide that the upper part of the distillation column comprise at least three successive stages, the first of which is the lowest, and that the fifth top fraction be introduced above the second stage.
The invention may also provide that the upper part of the distillation column comprise at least two successive stages, the first of which is the lowest, and that the second sample fraction be introduced above the first stage.
The invention may also provide that the upper part of the distillation column comprise at least three stages, the first of which is the lowest, into which the first sample fraction is introduced in a lower part of the first stage, and that the third top fraction be introduced below the last stage.
Finally, the invention may provide that the third top fraction be introduced into the first stage of the upper part of the distillation column.
The invention may also provide that the middle part of the distillation column comprise at least two successive stages, the first of which is the lower, and in which the first bottom fraction is introduced at least into the first stage, and that the first top fraction is introduced above the first stage.
According to a second aspect, the invention concerns a methane-rich gas obtained by the present procedure, as well as a liquefied gas which is rich in C2 and higher hydrocarbons, obtained by the present process.
According to a third aspect, the invention concerns an installation for separation of a cooled mixture under pressure containing methane and C2 and higher hydrocarbons, into a final light methane-rich fraction and a final heavy fraction rich in C2 and higher hydrocarbons, comprising means to carry out a first stage in which the mixture cooled under pressure is separated, in a first flask, into a relatively more volatile first top fraction and a relatively less volatile first bottom fraction, in which the first bottom fraction is introduced into a middle part of a distillation column, in which the final heavy fraction rich in C2 and higher hydrocarbons is collected in the lower part of the column as the second bottom fraction, and in which there is introduced the first top fraction into an upper part of the distillation column, after it has undergone pressure reduction in a turbine; in which a second methane-rich top fraction is collected in the upper part of this column, in which the second top fraction then undergoes compression and cooling to obtain the final light fraction, and in which a first sample fraction is removed from the final light fraction; this installation comprising means to carry out a second stage in which the first sample fraction is introduced, after cooling and liquefaction, into the upper part of the distillation column, this installation comprising means to carry out a third stage in which the first bottom fraction is subjected to a number of sub-stages including reheating, passage into a second flask, and separation into a third relatively more volatile top fraction, and a third relatively less volatile bottom fraction, in which the third bottom fraction is introduced into the middle part of the distillation column, and in which the third top fraction, after cooling and liquefaction, is introduced into the upper part of the distillation column.